Considering the Effects of Gravity When Developing and Field Testing Planetary Excavator Robots

Abstract

One of the challenges of field testing planetary rovers on Earth is the difference in gravity between the test and the intended operating conditions. This not only changes the weight exerted by the robot on the surface but also affects the behaviour of the granular surface itself, and unfortunatly no field test can fully address this shortcoming. This research introduces novel experimentation that for the first time subjects planetary excavator robots to gravity offload (a cable pulls up on the robot with 5/6 its weight, to simulate lunar gravity) while they dig. Excavating with gravity offload underestimates the detrimental effects of gravity on traction, but overestimates the detrimental effects on excavation resistance; though not ideal, this is a more balanced test than excavating in Earth gravity, which underestimates detrimental effects on both traction and resistance. Experiments demonstrate that continuous excavation (e.g. bucket-wheel) fares better than discrete excavation (e.g. front-loader) when subjected to gravity offload, and is better suited for planetary excavation. This key result is incorporated into the development of a novel planetary excavator prototype. Lessons learned from the prototype development also address ways to mitigate suspension lift-off for lightweight skid-steer robots, a problem encountered during mobility field testing.